Method and system for radial and tangential tilt calibration of optical storage systems

ABSTRACT

A system and method for adjusting the radial tilt, tangential tilt, or a combination of radial and tangential tilt of an optical detection unit in an optical disc reading system can include applying different weighting factors to different signal components depending on which detection area detects the component, measuring a value of a signal characteristic, such as signal-to-noise ratio, of two signals with different sets of weighting factors, and determining an adjustment factor to the radial tilt as a function of the of the measured signal characteristic values.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. provisionalapplication Ser. No. 60/914,473 filed on Apr. 27, 2007. The disclosureof the provisional application is incorporated herein by reference inits entirety.

FIELD OF THE DISCLOSURE

Aspects of the present invention relate generally to optical discsystems for storing digital data, and more particularly to a method andsystem for calibrating the radial and/or tangential tilt of the opticaldetection unit in an optical disc reading system.

BACKGROUND

Optoelectronic storage media such as compact discs (CDs) and digitalversatile discs (DVDs) are commonly used for storing digital data. Atypical disc is a little over one millimeter thick and is predominantlymade out of a material such as plastic. Data is coded onto the disc bycreating a series of bumps in the plastic. The plastic can then becovered by a reflective material such as aluminum and a protectivematerial such as acrylic. The series of bumps on the disc form a datatrack that begins at the center of the disc and spirals outward. In aCD, for example, the bumps may be smaller and may be spaced differentlythan in a DVD, high-density DVD (HD DVD), or Blu-ray Disc.

FIGS. 1 a and 1 b show a typical laser 101 and optical detection unit102 in one type of optoelectronic processing device. The laser 101directs a beam of light 103 towards a disc 104. If the beam of light 103strikes a bump 105 as in FIG. 1 a, then it might be reflected towardsthe optical detection unit (ODU) 102. If the beam of light 104 strikeswhere there is not a bump, sometimes referred to as a land 106, as inFIG. 1 b, then the beam might be reflected away from the ODU 102. Thechanges in reflection can be transmitted as bits of digital data by theODU 102 to processing circuitry 107 which processes the received dataand provides an appropriate output, depending for example on whether theoutput is to be music, video, or another type of data.

DVDs, HD DVDs, China Format HD Discs (C-HD), Blu-ray Discs, and otheroptical disc-reading systems work using the same principles as the CDsystem described above, but they utilize lasers of a smaller wavelengththan CDs that allow the bumps and lands to be smaller and spaced moreclosely together, thus allowing for more data to be stored on a disc ofthe same physical size. FIG. 2 shows an example of an optical disc 210that may be used in such a system. The disc 210 contains a hole 211 atthe center so that a drive motor can rotate it in the direction shown bythe arrow 212. As the disc 210 rotates, a head assembly 213 containingan optical detector focuses on the reflection of a laser from the discsurface 210. In order to properly focus the optical detector, thedisc-reading device must move the head assembly depending on thelocation of the data track being read. Because the bumps on the discsurface 210 are so small and must be read in rapid succession, the headassembly 213 must be able to move with extreme precision and achievefocus rapidly. Accordingly, the head assembly 213 is configured to movein two different manners. First, it can slide on an arm 214 along thex-axis, and second, it can make minor adjustments to its focus byrotating along the y-axis, thus changing the angle of the opticaldetector relative to the x-axis. This angle is commonly referred to asthe radial tilt.

It is also common for the head assembly 213 to include a means, such asa drive motor, for rotating about the x-axis in order to adjust for atilt in the disc 210 or a tilt of the optical detector. The angle of theoptical detector relative to the y-axis is commonly referred to as thetangential tilt. In a typical system, both radial tilt and tangentialtilt may be present.

Several methods exist in the art for calibrating the radial andtangential tilt. One such method includes sweeping a range of angles todetermine which angle produces the fewest errors. Another methodincludes making incremental changes to the tilt based on whether signalquality, as judged by a signal characteristic such as signal-to-noiseratio, is improving or worsening. Both these methods, however, areundesirable because they are slow to arrive at a desired tilt angle. Amore sophisticated method can be implemented that measures asignal-to-noise-ratio over separate and independent portions of a signaland uses the difference between those portions to calculate a desiredtilt. Such a method may be faster than other methods known in the art,but it is still undesirable because it requires monitoring multipleindependent variables, and as a result can require multiple datachannels when configured into hardware, thus increasing the complexityand cost of implementation.

Therefore, there exists in the art a need for a new method and systemfor accurately and rapidly calibrating the radial and/or tangential tiltof an optical detection unit in an optical disc reading system.Additionally, there exists in the art a need for such a system that canbe easily and inexpensively implemented into the hardware of an opticaldisc reading system.

SUMMARY

A device embodying aspects of the present invention can include anoptical disc reading system containing an optical detection unit withmultiple detection areas for detecting a signal reflected from the faceof an optoelectronic storage medium. The optical disc reading system canbe configured to apply different weighting factors to different signalcomponents depending on which detection area detects the component. Thesystem can further measure values of a signal characteristic, such assignal-to-noise ratio, of two signals with different sets of weightingfactors, and determine an adjustment factor to the radial tilt as afunction of the of the measured signal characteristic values. Based onthe adjustment factor, the radial tilt of the optical detection unit canbe altered. The same general method can be applied to tangential tilt,or to a combination of radial tilt and tangential tilt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show examples of a system using a laser and an opticaldetector to read the bumps on a disc and transmit those bumps as digitaldata.

FIG. 2 shows an example of a system with a disc and a movable opticaldetection unit configured to read the disc.

FIG. 3 shows an example of an optical detection unit configured toimplement aspects of the present invention.

FIG. 4 shows a flow chart illustrating a method embodying aspects of thepresent invention.

DETAILED DESCRIPTION

Aspects of the present invention include a novel method and system forcalibrating radial tilt and/or tangential tilt in optical storagesystems. Optimizing the tilt angle can result in better signal quality,and as a result, fewer errors. A method and system embodying aspects ofthe present invention can rapidly determine a desirable tilt angle withthe use of minimal extraneous hardware. Although some of thedescriptions of embodiments may only discuss radial tilt, the samesystems and methods described can also be applied to tangential tilt ora combination of radial and tangential tilt without the use of inventivefaculty.

FIG. 3 shows an example of an optical detection unit (ODU) 321 thatmight be used in a system or method embodying aspects of the presentinvention. The ODU 321 can have multiple detection areas. For example,the ODU 321 of FIG. 3 has four photo diodes 322 a-d. The signal detected(RF) by the ODU 321 can be the sum of the signals detected by the fourphotodiodes 322 a-d (i.e. RF=A+B+C+D). When reading a disc, the ODU 321can be oriented such that the radial direction is pointed towards thecenter of the disc, and the data track on the disc moves over the ODU321 in the tangential direction.

The ODU 321 of FIG. 3 can be connected to a mechanism (not shown) foradjusting the ODU's 321 tilt in the radial direction by rotating it onits tangential axis (the y-axis). It is desirable to be able to adjustthe radial tilt of the ODU 321 while still reading the data from thedisc, i.e., without losing channel lock. An aspect of the presentinvention includes determining a desired radial tilt as a function ofthe balance of the individual photodiodes 321 a-d. The balance of thephotodiodes can be determined by calculating a partial responsesignal-to-noise ratio (PRSNR) for the signal detected by two photodiodes on one side of the y-axis (for example, 322 a and 322 d) andcomparing it to the PRSNR for the signal detected by two photodiodes onthe other side of the y-axis (for example, 322 b and 322 c). Once thetwo PRSNRs have been measured, a normalized difference of PRSNR (NDP)can be determined as follows:

${{NDP} = \frac{{{PRSNR}\; 2} - {{PRSNR}\; 1}}{{{PRSNR}\; 1} + {{PRNSR}\; 2}}},{where}$PRSNR1=SNR(A, D) and PRSNR2=SNR(B, C).PRSNR1 is the SNR of the signal detected by photodiodes 322 a and 322 d,and PRSNR2 is the SNR of the signal detected by photodiodes 322 b and322 c.

NDP has an approximately linear correlation to radial tilt angle, andmaximum SNR occurs at approximately the same tilt angle as where NDP=0.Therefore, once a value for NDP has been determined, a desired tiltangle can also be determined based on the linear relationship betweenNDP and tilt angle. The linear relationship between NDP and tilt anglecan be determined by a system designer and can be built into thehardware or software of a system. Based on the desired tilt angle, thecurrent tilt angle of the ODU can be adjusted to the desired tilt angle.

A further aspect of the present invention includes measuring NDP bydetermining the SNR of signals with weighted values for A, B, C, and D.For example, instead of determining values for PRSNR from the signalsRF1=A+D and RF2=B+C, an aspect of the present invention includescalculating PRSNR1 and PRSNR2 from the following signals:RF1(α₁)=(2−α₁)(A+D)+α₁(B+C), where 0≦α₁≦2andRF2(α₂)=(2−α₂)(A+D)+α₂(B+C), where 0≦α₂≦2

The NDP can be calculated as discussed above with PRSNR1 equal to themeasured SNR of R1(α₁) and PRSNR2 equal to the measured SNR of RF2(α₂).Based on the determined NDP, a desired tilt angle can also be determinedas discussed above.

Measuring values of PRSNR1 and PRSNR2 with unweighted values for A, B,C, and D (i.e. RF1=A+D and RF2=B+C) presents one of two challenges.Either the system will have to utilize one channel for passing data fromthe ODU to processing circuitry and rely on the signals of A+D and B+Cindividually to maintain data lock, or the system will have to have twodata channels so that it can maintain data lock with a separate signal(for example, RF=A+B+C+D) while it measures PRSNR for RF1 and RF2.Relying on RF1=A+D and RF2=B+C to maintain data lock is an undesirablesolution because those signals only have a portion of the full RF signaland may not be strong enough to maintain data lock, resulting inundesirable delays in processing data. While having a second channel canovercome this shortcoming, it too is undesirable because it requirescostly additional hardware.

In order to avoid the problems discussed above, a system designer canchoose values of α₁ and α₂ such that the signals RF1(α₁) and RF2(α₂)have sufficiently high SNR to maintain channel lock, thus avoiding theneed for a second channel. For example, a system designer might chooseα₁ and α₂ based on the following parameters: α₁=2−α₂ and 0.5<α₂<1.

FIG. 4 is a flow chart showing an example of a method embodying aspectsof the present invention. The method begins when a system such as aBlu-ray Disc player is turned on or put into a play mode (block 400). Anoptical detection unit is positioned relatively to a data track on adisc (block 410). Once positioned, a value for PRSNR1 can be measured(block 420), and a value for PRSNR2 can be measured (block 430). Fromthe values of PRSNR1 and PRSNR2, a value of NDP can be calculated (block440). An adjustment to the radial tilt can be determined based on thecalculated value of NDP (block 450). Once the adjustment to the radialtilt has been made, the method can finish (block 460).

For ease of discussion, the foregoing description has explained aspectsof the present invention in terms of measuring SNR and partial responseSNR. It will be readily apparent to an ordinary skilled artisan,however, that metrics other than SNR, such as metrics that are eitherproportional or inversely proportional to error rate, can also be usedwithout deviating from the spirit of the present invention and withoutthe use of inventive faculty. For example, a system designer mightchoose to measure a Viterbi margin metric (VMM) rather than PRSNR. VMMis generally a measure of how many times the path metric between theselected path and the next best path in the Viterbi detector is smallerthan a certain threshold. The smaller VMM is, the better the channel is.Using VMM, it is possible to determine NDP as follows:

${NDP} = \frac{{{VMM}\; 1} - {{VMM}\; 2}}{{{VMM}\; 1} + {{VMM}\; 2}}$where VMM1 is approximately inversely proportional to PRSNR1 and VMM2 isinversely proportional to PRSNR2.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art, and the generic principles and specific examplesdefined herein may be applied to other embodiments without the use ofinventive faculty. For example, some or all of the features of thedifferent embodiments discussed above may be deleted from theembodiment. Therefore, the present invention is not intended to belimited to the embodiments described herein but is to be accorded thewidest scope defined only by the claims below and equivalents thereof.

1. A method of adjusting a tilt of an optical detection unit (ODU) withmultiple detection areas on opposite sides of an axis, the methodcomprising: receiving a first set of signal components (A₁, B₁, C₁, andD₁) at respective detection areas of the ODU; generating a first signal(RF₁) equal to x₁*A₁+x₂*B₁+x₃*C₁+x₄*D₁, wherein x₁, x₂, x₃, and x₄ areweighting factors greater than zero; measuring a first value for asignal characteristic of RF₁; receiving a second set of signalcomponents (A₂, B₂, C₂, D₂) at respective detection areas of the ODU;generating a second signal (RF₂) equal to y₁*A₂+y₂*B₂+y₃*C₂ y₄*D₂,wherein y₁, y₂, y₃, and y₄ are weighting factors greater than zero;measuring a second value for the signal characteristic of RF₂;calculating an amount of tilt adjustment as a function of the firstvalue and the second value; and, adjusting the tilt of the ODU based onthe amount of tilt adjustment.
 2. The method of claim 1, wherein theaxis is a tangential axis, and the tilt is a radial tilt.
 3. The methodof claim 1, wherein the axis is a radial axis, and the tilt is atangential tilt.
 4. The method of claim 1, wherein the detection areasfor A1 and D1 are located on one side of the tangential axis and thedetection areas for B₁ and C₁ are located on the other side oftangential axis.
 5. The method of claim 1, wherein the signalcharacteristic is a signal-to-noise ratio (SNR).
 6. The method of claim1, wherein x₁ is equal to x₄, and x₂ is equal to x₃.
 7. The method ofclaim 1, wherein x₁ is equal to y₂, and y₁ is equal to x₂.
 8. The methodof claim 1, wherein 0.5<x₁<1, and x₂=2−x₁.
 9. The method of claim 1,wherein the signal characteristic is a Viterbi margin metric.
 10. Anoptical disc reading system comprising: an optical detection unit (ODU)with first and second detection areas on a first side of an axis andthird and fourth detection areas on a second side of the axis; circuitryto create a first signal (RF₁) equal to x₁*A₁+x₂*B₁+x₃*C₁+x₄*D₁, whereinA₁, B₁, C₁, and D₁ are signal components received at respectivedetection areas of the ODU, and x₁, x₂, x₃, and x₄ are weighting factorsgreater than zero; circuitry to create a second signal (RF₂) equal toy₁*A₂+y₂*B₂+y₃*C₂+y₄*D₂, wherein A₂, B₂, C₂, and D₂ are signalcomponents received at respective detection areas of the ODU, and y₁,y₂, y₃, and y₄ are weighting factors greater than zero; circuitry tomeasure a first value of a signal characteristic of RF₁ and a secondvalue of the signal characteristic of RF₂; circuitry to calculate a tiltadjustment factor as a function of the first value and the second value;and, circuitry to adjust a tilt of the optical detection unit based onthe tilt adjustment factor.
 11. The system of claim 10, wherein the axisis a tangential axis, and the tilt is a radial tilt.
 12. The system ofclaim 10, wherein the axis is a radial axis, and the tilt is atangential tilt.
 13. The system of claim 10, wherein the signalcharacteristic is a signal-to-noise ratio (SNR).
 14. The system of claim10, wherein A₁ is a signal component detected in the first detectionarea, D₁ is a signal component detected in the second detection area, B₁is a signal component detected in the third detection area, and C₁ is asignal component detected in the fourth detection area.
 15. The systemof claim 10, wherein x₁=x₄, and x₂=x₃.
 16. The system of claim 10,wherein x₁=y₂, and y₁=x₂.
 17. The system of claim 10, wherein 0.5<x₁<1,and x₂=2−x₁.
 18. The system of claim 10, wherein the signalcharacteristic is a Viterbi margin metric.
 19. A method of adjustingradial tilt and tangential tilt of an optical detection unit (ODU) withmultiple detection areas, the method comprising: receiving a first setof signal components (A₁, B₁, C₁, and D₁) at respective detection areason opposite sides of a tangential axis of the ODU; generating a firstsignal (RF₁) equal to x₁*A₁+x₂*B₁+x₃*C₁+x₄*D₁, wherein x₁, x₂, x₃, andx₄ are weighting factors greater than zero; measuring a first value fora signal characteristic of RF₁; receiving a second set of signalcomponents (A₂, B₂, C₂, D₂) at respective detection areas on oppositesides of the tangential axis of the ODU; generating a second signal(RF₂) equal to y₁*A₂+y₂*B₂+y₃*C₂+y₄*D₂, wherein y₁, y₂, y₃, and y₄ areweighting factors greater than zero; measuring a second value for thesignal characteristic of RF₂; calculating an amount of radial tiltadjustment as a function of the first value and the second value; and,adjusting the radial tilt of the ODU based on the amount of tiltadjustment; receiving a third set of signal components (A₃, B₃, C₃, andD₃) at respective detection areas on opposite sides of a radial axis ofthe ODU; generating a third signal (RF₃) equal tow₁*A₃+w₂*B₃+w₃*C₃+w₄*D₃, wherein w₁, w₂, w₃, and w₄ are weightingfactors greater than zero; measuring a third value for a signalcharacteristic of RF₃; receiving a fourth set of signal components (A₄,B₄, C₄, D₄) at respective detection areas on opposite sides of theradial axis of the ODU; generating a fourth signal (RF₄) equal toz₁*A₄+z₂*B₄+z₃*C₄+z₄*D₄, wherein z₁, z₂, z₃, and z₄ are weightingfactors greater than zero; measuring a fourth value for the signalcharacteristic of RF₄; calculating an amount of tilt adjustment as afunction of the third value and the fourth value; and, adjusting thetangential tilt of the ODU based on the amount of tilt adjustment.